linux-kernel - Re: [patch 1/2] c/r: prctl: Add ability to set new mm_struct::exe

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Date:	Fri, 23 Mar 2012 10:06:14 -0700
From:	Matt Helsley <matthltc@...ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	richard -rw- weinberger <richard.weinberger@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Kees Cook <keescook@...omium.org>, Tejun Heo <tj@...nel.org>,
	Matt Helsley <matthltc@...ibm.com>
Subject: Re: [patch 1/2] c/r: prctl: Add ability to set new
 mm_struct::exe_file

On Thu, Mar 22, 2012 at 04:38:43PM -0700, Eric W. Biederman wrote:
> Cyrill Gorcunov <gorcunov@...nvz.org> writes:
> 
> > On Mon, Mar 19, 2012 at 03:46:49PM -0700, Andrew Morton wrote:
> >> > >>
> >> > >> What is this mysterious "security reason"?
> >> > >>
> >> > >
> >> > > Oh, sorry I should have included Matt's comment here
> >> 
> >> Please send a patch with the updated changelog and improved comment?
> >> 
> >
> > Andrew, take a look please, will the changelog and comments look
> > better?
> 
> Can you change this to take an actual address and get the exe_file
> from an mmapped area and make certain that the mmaped_area is already
> mapped MAP_EXEC.

Do you mean PROT_EXEC/VM_EXEC?

> 
> That will prevent out-right lies.
> 
> At least then we will know that exe_file will at least be a file that is
> mapped executable in the process's address space.  It's not a lot better
> but it makes /proc/<pid>/exe at almost as trustable as it is now.

I don't dislike the idea. However just because it's mapped with one of
those flags does not mean that a single instruction of it will ever be
executed. So it's not much better than using the fd :/.

Perhaps there is some way to use the userspace stack and/or regs to get
a reasonable instruction pointer, lookup its VMA, and use that? I'm not
sure that would work for c/r though...

Cheers,
	-Matt

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